Using the Launcher

Once you download a launcher, you'll see a screen that asks you where you want to install the game. If you point this at where your game is currently installed, you won't need to download the whole client again.

EDIT:

If you are seeing "Support Package Install... Failed 1010" the launcher should fix it automatically. If the last line says "Locating Support Package... Downloading" just let it run.

EDIT: Finding your install location

If you still have the shortcut icon you used to launch PotBS on your desktop, right click on it and select Properties. The install directory is the one listed in the "Start in:" field.